Typical examples of an infrared signal processing circuit are: remote controllers of home electric appliances and peripheral devices of personal computers, each of which performs data communication in compliance with IrDA (Infrared Data Association) standard or IrDA Control standard.
For example, a conventional infrared remote control receiver 110 includes a photodiode chip 101 and a reception chip 108 as shown in FIG. 23. The photodiode chip 101 converts a remote control transmission signal received from an infrared remote control transmitter (not shown) into a current signal Iin. The reception chip 108 includes: a current-to-voltage-conversion circuit 102 for converting the current signal Tin having been generated into a voltage signal; an amplifying circuit 103 for amplifying the voltage signal having been generated; a bandpass filter circuit (hereinafter, BPF) 104 for extracting a carrier frequency component from the voltage signal having been amplified; a carrier detection circuit 105 for detecting a carrier from the carrier frequency component having been extracted; an integrating circuit 106 for integrating carrier-existing periods; and a hysteresis comparator 107 which compares an output of the integrating circuit 106 with a threshold level, thereby (i) judging whether or not the carrier exits and (ii) outputting the result of the judgment in the form of digital output. The digital output Dout of the hysteresis comparator 107 is sent to a microcomputer or the like which controls an electronic device.
FIG. 24 shows an output of each circuit of the infrared remote control receiver 110. FIG. 24(a) shows an output of the current signal Iin. FIG. 24(b) shows an output of the BPF 104 (solid line) and that of the carrier detection circuit 105 (dotted line). FIG. 24(e) shows an output of the integrating circuit 106 (solid line). FIG. 24(d) shows a digital output Dout of the infrared remote control receiver 110.
Note that the dotted line in FIG. 24(c) is a threshold level.
The remote control transmission signal is an ASK (Amplitude Shift Keying) signals (remote controller transmission signals) modulated by a predetermined carrier of, for example, approximately 30 kHz to 60 kHz. This carrier component of 30 kHz to 60 kHz also exists in light from a home-use inverter fluorescent light. Accordingly, an infrared remote control receiver 110, when used around a fluorescent light, may malfunction by detecting noise stemming from the fluorescent light. In worst situation, the infrared remote control receiver 110 may not be able to accurately receive signals transmitted from the remote control.
To reducing the noise from an inverter fluorescent light, the Q-value of the BPF 104 is increased thereby increasing the carrier selectivity. However, raising of the Q-value of the BPF 104 causes distortion in a waveform of the BPF 104 and an increase in the pulse width. This is described in detail hereinbelow.
As shown in FIG. 25, the BPF 104 includes: transconductance amplifiers (hereinafter, simply referred to as GMs) 111 and 112; an attenuator (ATT) 113 (damping ratio: 1/α); and capacitors C11 and C12. The transfer function H(s) of the BPF 104 is expressed by the following Formula (1).
According to Kirchhoff's law,gm 111*(−vo)=s*C11*(v1−vin)gm 112*(v1−(R112/(R111+R112))*vo)=s*C12*vo. 
By eliminating V1,H(s)=(H*ω0/Q*s)/(s2+ω0/Q*s+ω02)  (1)ω0=((gm 111*gm 112)/(C11*C12))1/2 Q=α*((C12*gm 111)/(C11*gm 112))1/2 H=α,
where:
vin is an input voltage of the BPF 104;
vo is an output voltage of the BPF 104;
i112 is an output current of the GM111;
i112 is an output current of the GM112;
v1 is an output voltage of the GM111;
gm 111 is a transconductance of the GM111;
gm 112 is a transconductance of the GM112;
C11 is a capacitance value of the capacitor C11;
C12 is a capacitance value of the capacitor C12;
R111 is an output impedance of the GM111;
R112 is an output impedance of the GM112;
ω0 is a natural angular frequency;
H is the gain; and
s is a complex number.
A sine wave response of the BPF 104 is obtained as follows. Namely, where Laplace transform of sine wave is as presented in Formula (2), the sine wave response of the BPF is obtained by performing reverse-Laplace transform of H(S)F(S) (Formula (3)).F(s)=L(sin(ω0t))=ω0/(s2+ω02)2)  (2)
                                          H            ⁡                          (              s              )                                *                      F            ⁡                          (              s              )                                      =                ⁢                                            (                              H                *                ω                ⁢                                                                  ⁢                                  0                  /                  Q                                *                s                            )                        /                          (                                                s                  2                                +                                  ω                  ⁢                                                                          ⁢                                      0                    /                    Q                                    *                  s                                +                                  ω                  ⁢                                                                          ⁢                                      0                    2                                                              )                                *          ω          ⁢                                          ⁢                      0            /                          (                                                s                  2                                +                                  ω                  ⁢                                                                          ⁢                                      0                    2                                                              )                                                              =                ⁢                                            (                                                -                  H                                *                ω                ⁢                                                                  ⁢                0                            )                        /                          (                                                s                  2                                +                                  ω                  ⁢                                                                          ⁢                                      0                    /                    Q                                    *                  s                                +                                  ω                  ⁢                                                                          ⁢                                      0                    2                                                              )                                +                                                ⁢                              (                          H              *              ω              ⁢                                                          ⁢              0                        )                    /                      (                                          s                2                            +                              ω                ⁢                                                                  ⁢                                  0                  2                                                      )                                                  =                ⁢                                            (                                                -                  H                                *                ω                ⁢                                                                  ⁢                0                            )                        /                          {                                                                                                                                            (                                                      s                            +                                                          ω                              ⁢                                                                                                                          ⁢                                                              0                                /                                                                  (                                                                      2                                    *                                    Q                                                                    )                                                                                                                                              )                                                2                                            +                                                                                                                                                          (                                                  (                                                      ω                            ⁢                                                                                                                  ⁢                            0                            ⁢                                                                                          (                                                                                                      (                                                                                                                  4                                        *                                                                                  Q                                          2                                                                                                                    -                                      1                                                                        )                                                                    /                                                                      (                                                                          4                                      *                                                                              Q                                        2                                                                                                              )                                                                                                  )                                                                                            1                                /                                2                                                                                                              )                                                )                                            2                                                                                  }                                +                                                ⁢                              (                          H              *              ω              ⁢                                                          ⁢              0                        )                    /                                    (                                                s                  2                                +                                  ω                  ⁢                                                                          ⁢                                      0                    2                                                              )                        .                              When: ((4*Q2−1)/(4*Q2))1/2≈1,=(−H*ω0)/{(s+ω0/(2*Q))2+ω02}+(H*ω0)/(s2+ω02)
When the first term and the second term are subjected to a reverse-Laplace transform,L−1(H(s)F(s))=H*{(−exp(−ω0t/(2*Q))*sin(ω0t)+sin(ω0t))=H(1−exp(−ω0t/(2*Q)))*sin(ω0t)  (3)
The (1−exp(−ω0t/(2*Q))) in the Formula (3) affects the waveform distortion.
FIG. 26 shows outputs of the BPF 104. FIG. 26(a) shows an output of the BPF 104 when the Q-value is low. FIG. 26(b) shows an output of the BPF 104 when the Q-value is high. FIG. 26(c) shows an output of the BPF 104 when the Q-value is high, and when close-distance communication is performed with an infrared remote control transmitter. Note that each figure also shows a digital output Dout of the infrared remote control receiver 110.
From the Formula (3) and FIG. 26, it is apparent that raising the Q-value of the BPF 104 causes larger distortion in the waveform of the output from the BPF 104 and an increase in the pulse width. These phenomena are particularly noticeable when the pulse width of the base frequency of the remote control transmission signal is small. As shown in the figure, the digital output Dout is not properly output due to the distortion in the waveform of the output from the BPF 104, and the reception sensitivity is deteriorated. In view of this, the Q-value of the BPF 104 in general is set within a range of approximately 10 to 15.
Recently, a data volume to be transmitted increased due to an increase in the number of functions of a remote controller, while an amount of light emission is reduced to lower the power consumption. Under such circumstances, the pulse width of remote control transmission signals transmitted is shortened. In an infrared remote control receiver supporting such remote control transmission signals with a short pulse width, the above mentioned problem of failing to receive the remote control transmission signals takes place, particularly when the pulse width of an output from the BPF is increased due to an increase in the Q-value of the BPF. For example, in a case of an RC-MM (Remote Control-Multi Media Protocol), when the pulse width of a remote control transmission signal is 166 μsec, the pulse width of the signal in an infrared remote control receiver needs to fall within a range from 80 μsec to 275 μsec. The above mentioned problem is particularly considerable in short-distance communication in which signal amplitude is large as shown in FIG. 26(c).
To solve the problem, the inventors of the present invention has suggested the following infrared remote control receiver. In the infrared remote control receiver, an output of its BPF is detected and is compared with a several threshold levels. If the output surpasses the threshold levels, it is judged that noise of inverter fluorescent light has entered, and the pulse width of the BPF is increased. In such a case, the infrared remote control receiver performs control to decrease the gain and Q-value of the BPF, so as to reduce the noise of inverter fluorescent light and distortion in the waveform of the BPF output. In this case, a BPF capable of adjusting constants such as the Q-value or the gain is needed.
Further, the BPF has a single-ended input as shown in FIG. 25. Therefore, when power source noise or the like is overlapped at the input, the noise is amplified, because the gain is increased nearby the center frequency due to a BPF characteristic. Thus, a characteristic of removing power-source noise is deteriorated.
Further, to cancel inverter fluorescent light noise, the infrared remote control receiver may have a band-elimination filter circuit (hereinafter, BEF). As shown in FIG. 27, the BEF 130 includes: transconductance amplifiers (hereinafter, simply referred to as GMs) 121; an attenuator (ATT) 123 (damping ratio 1/α); and capacitors C21 and C22. The transfer function H(s) of the BEF 130 is expressed by the following Formula (4).H(s)=H*(s2+ωn2)/(s2+ω0/Q*s+ω02)  (4)ω0=ωn=((gm 121*gm 122)/(C21*C22))1/2 Q=α*((C22*gm 121)/(C21*gm 122))1/2 H=1
Where:
ω0 is a natural angular frequency;
ωn is a noise natural angular frequency;
H is the gain;
s is a complex number;
gm 121 is a transconductance of the GM121;
gm 122 is a transconductance of the GM122;
C21 is a capacitance value of the capacitor C21; and
C22 is a capacitance value of the capacitor C22.